Processes for converting lower oxygenates such as methanol and dimethyl ether to hydrocarbons are known and have become of great interest in recent times because they offer an attractive way of producing liquid hydrocarbon fuels, especially gasoline, from sources which are not of liquid petroliferous origin. In particular, they provide a way by which methanol can be converted to gasoline boiling range products in good yields. The methanol, in turn, may be readily obtained from coal by gasification to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas by other conventional processes.
The conversion of methanol and other lower aliphatic oxygenates to hydrocarbon products may take place in a fixed bed process as described in U.S. Pat. Nos. 3,998,899; 3,931,349 (Kuo) and 4,035,430. In the fixed bed process, the methanol is usually first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, dimethyl ether (DME) and water. This mixture is then passed at elevated temperature and pressure over a catalyst such as ZSM-5 zeolite for conversion to the hydrocarbon products which are mainly in the range of light gas to gasoline. Water may be removed from the methanol dehydration products prior to further conversion to hydrocarbons and the methanol can be recycled to the dehydration step, as described in U.S. Pat. No. 4,035,430. Removal of the water is desirable because the catalyst may tend to become deactivated by the presence of excess water vapor at the reaction temperatures employed; but this step is not essential.
In the operation of an adiabatic fixed bed process, a major problem is thermal balance. The conversion of the oxygenated feed stream (methanol, DME) to the hydrocarbons is a strongly exothermic reaction liberating approximately 1480 kJ. (1400 Btu) of heat per kilogram of methanol. In an uncontrolled adiabatic reactor this would result in a temperature rise which would lead to extremely fast catalyst aging rates or even to damage to the catalyst. Furthermore, the high temperatures which might occur could cause undesirable products to be produced or the product distribution could be unfavorably changed. It is therefore necessary that some method should be provided to maintain the catalyst bed within desired temperature limits by dissipating the heat of the reaction.
One method is to employ a light gas portion of the hydrocarbon product as recycle, as described in U.S. Pat. No. 3,931,349 (Kuo). Typically, cooled light hydrocarbon gas, rich in methane, ethane, etc., is separated from the gasoline and LPG products, re-compressed and reheated before being mixed with the reactant feedstream entering the bed of conversion catalyst. Although effective in controlling bed temperature, the expense of cooling the recycle gas, compressing it and re-heating it add to the cost of the conversion, indicating that a reduction in recycle ratio would be economically desirable. The recycle ratio can indeed be decreased but only with certain disadvantages. Not only will the temperature rise across the catalyst bed be greater, thereby increasing the aging rate of the catalyst but, in addition, the reactor must be operated at a lower and generally less favorable temperature; the outlet temperature must be lowered in order to protect the catalyst from the increased partial pressure of the water which is consequent upon the lower partial pressure of the recycle gas and the inlet temperature must be lowered even further in order to compensate for the greater temperature rise across the catalyst bed. This is generally undesirable because the octane number of the gasoline product is related to reactor temperature with the higher octane products being produced at the higher temperatures. There is also a minimum reactor inlet temperature that must be maintained for the conversion to proceed and consequently, there is a limit on the extent to which the recycle ratio can be reduced.
A similar proposal is set out in U.S. Pat. No. 4,404,414. The process described in this patent employs a number of fixed bed reaction zones in which oxygenated feedstock is converted to hydrocarbon products by means of contact with a conversion catalyst. The temperature in the reactors is maintained at the desired value by the use of a diluent which is passed through the reactors in sequence before it is completely cooled and separated from the conversion products. The diluent in this case is light hydrocarbon gases which have been separated from the liquid hydrocarbon products and water. Once again, the expense of cooling the recycle gas, compressing it and re-heating it add to the cost of the conversion.